Methanol detection in M 82

Abstract

Context.The nuclear starburst region in M 82 shows systematical low abundances of some complex molecules when compared with other starburst galaxies. This is likely related to a presumably photodissociation dominated environment. In particular, methanol is known to show relatively low abundance because it is easily photodissociated.Aims.We present a multilevel study of the emission of methanol, detected for the first time in this galaxy, and discuss the origin of its emission.Methods.Observations of three transitions of CH3OH towards the center and two positions around the nucleus of M 82 are presented. Two different components are found, one with high excitation (, K) and the other with low excitation (, K).Results.The high observed methanol abundance of a few 10-9 can only be explained if injection of methanol from dust grains is taken into account. While the overall [ CH3OH] /[ NH3] ratio is much larger than observed towards other starbursts, the dense high excitation component shows a similar value to that found in NGC 253 and Maffei 2. Conclusions.Our observations suggest the molecular material in M 82 to be formed by dense warm cores, shielded from the UV radiation and similar to the molecular clouds in other starbursts, surrounded by a less dense photodissociated halo. The dense warm cores are likely the location of recent and future star formation within M 82.